Polyester resins derived from terephthalic acid and reactive derivatives thereof, such as dimethyl terephthalate, and alkanediols of from, e.g., 2 to 10 inclusive carbon atoms, e.g., ethylene glycol and 1,4-butanediol as well as related diols, such as cyclohexane dimethanol, and mixtures of such resins, have been known for some time and have become important constituents in injection moldable compositions. Workpieces molded from such polyester resin compositions, alone, or combined with reinforcements, offer a high degree of surface hardness and abrasion resistance, high gloss and lower surface friction. More recently, blends of such polyester resins with one or more second resins have become of significant commercial interest because such second resins, carefully selected, can greatly improve impact strength, as well as tensile strength, modulus and distortion temperature under load in parts molded from such compositions. Such second resins can comprise aromatic polycarbonate resins, alone, or in further combination with other resins, such as polyacrylate resins. See, for example, Cohen and Dieck, U.S. Pat. No. 4,257,937.
One major problem in producing such blends, however, is that of chemical incompatibility under the typical, but harsh, thermal processing conditions. This is known to lead to variability in the final fabricated articles, e.g., injection molded parts. The problem seems to be related to a known type of ester interchange between esters and carbonates. See, e.g., J. Devaux, P. Godard, and J. P. Mercier, West German Patent Publication No. 27-10-729, Sept. 22, 1982, who stabilized polycarbonate - poly(1,4-butylene terephthalate) blends with phosphites, especially di-n-octadecylphosphite and tri-phenyl phosphite. In the case of the aforesaid blend, the effect of the interchange can easily be seen by changes in the melting point. As the ester-carbonate exchange process becomes more pronounced, the crystalline melting point (Tm) of the poly(1,4-butylene terephthalate) decreases. Simpler compounds, e.g., diesters of organic acids and reaction products of phosgene or a phosgene precursor with mono-ols, such as diphenyl carbonate also undergo such interchange, but in these cases, heating yields increasing amounts of by-products, such as phenyl benzoate, as the ester exchange proceeds, and measurement of by-product evolution becomes the method of choice for following the reaction. In any event, the use of phosphites as stabilizers is not entirely satisfactory because they have a tendency to be unstable to both hydrolysis and oxidation thereby often giving irreproducible and unreliable results.
It has now been discovered that a specific inorganic boron compound, namely, boric acid, is highly effective to stabilize such ester/carbonate compositions. Boric acid, a rather innocuous midly acidic reagent, is stable and can be used in various manners, including prior incorporation as a concentrate in the ester compound, or at an appropriate level in the carbonate compound. In addition to the high degree of reliability as stabilizers in such compositions, boric acid does not deterimentally affect any of the components in the compositions, especially resinous components, e.g., polyesters or polycarbonates.
Under some interpretations of the state of the art, one might conclude (i) that the use of boric acid is unpatentably obvious, and (ii) the interaction problem does not exist absent the application of elevated temperatures, and therefore claims which do not call for heat do not distinctly point out an invention. Both such conclusions would be erroneous.
As to (i), although Fritz, et al., U.S. Pat. Nos. 321,435; Cohen, et al., 4,257,937, and Nassar, et al., J. Appl. Polymer Science, Vol. 23, 85-99 (1979), might imply that there is no ester interchange reaction between polyesters and polycarbonates, and therefore boric acid functions by some other mechanism than that which is claimed, and for such stabilization boric acid is prima facie obvious, other, more authoritative sources do establish that thermally-induced ester interchange does occur, for example Doerr, U.S. Pat. Nos. 3,752,866, cited in Kawase, et al., 3,953,539, and two affidavits filed in the Official Patent Office File of Kawase, et al. on Feb. 28, 1975 and Aug. 25, 1975, respectively. Attention is also directed to Seymour, et al., U.S. Pat. No. 4,088,709. In any event, nothing in the prior art will show that boric acid has ever been used before to prevent ester interchange between polyesters and polycarbonates. Moreover, its use for this purpose is patentably unobvious because Kawase, et al. found that phosphorus compounds were uniquely effective to prevent such interchange for butylene terephthalate resins, but not for ethylene terephthalate resins whereas as will be shown in the examples hereinafter, particularly Example 5, boric acid reduces ester interchange with 2-carbon alkylene esters, as well as 4-carbon alkylene esters (Example 1). These facts demonstrate conclusively that boric acid unexpectedly provides a wider range of compositions stabilized against thermally-induced ester interchange reactions.
Especially difficult to melt stabilize are combinations of polyester resins with polycarbonates and third resins such as polyacrylates, as well as flame retarded blends of polyesters and polycarbonates. Such formulations are rendered unprecedentedly melt stable with boric acid, according to the present invention. In addition to the specific instances noted above, stabilization can also be induced in other combinations of polyesters and other resins, especially those in which an active catalyst was used to prepare one or all of the polymers in the blend.